In some wireless applications, a power amplifier must be able to produce a range of output power levels. For example, in a cellular telephone environment, a base station may dictate the power level at which each cell phone should transmit (based on factors such as the physical distance from the base station, for example). A critical performance metric for power amplifiers in this type of environment relates to efficiency, as defined as the ratio of power delivered to a load to the power drawn by the power amplifier. Most prior art power amplifier architectures are most efficient near their maximum output power levels and their efficiency decreases rapidly as the output power level decreases.
FIG. 1 is a schematic diagram of one example of a prior art power amplifier. FIG. 1 shows a power amplifier 100 including an output stage 102 and a transformation network 104. An RF input signal Rfi1 is provided to the output stage 102. The output stage 102 includes transistors M1 and M2, which form a push-pull amplifier. The output stage 102 provides a signal at node vs to the input to the transformation network 104. The transformation network 104 is comprised of a capacitor Cb, inductor Lt, and capacitor Ct, whose values are chosen to produce a desired output impedance at the transmit frequency. The transformation network provides an RF output vo to a load Rl.
In the example shown in FIG. 1, the inductor Lt and capacitor Ct transform the load impedance into a new impedance,
                                          Z                          i              ⁢                                                          ⁢              n                                =                                    L              t                                                      C                t                            ⁢                              R                t                                                    ,                            (        1        )            and the average power delivered to the load is
                                          P            l                    =                                    V              s              2                                      Z                              i                ⁢                                                                  ⁢                n                                                    ,                            (        2        )            where Vs represents the root mean square voltage present on node vs. In the example shown in FIG. 1, capacitor Cb is used to isolate the DC level on node vt from that of node vs, but does not change the transformed impedance significantly.
FIG. 2 is a plot illustrating the efficiency versus the output power of a typical prior art power amplifier, such as the amplifier shown in FIG. 1. As shown, as the output power level decreases, the efficiency also decreases. Since some power amplifiers (such as power amplifiers used in wireless transmission systems) spend a significant portion of time transmitting at low power levels, it is desirable to increase the efficiency at low power levels.